Increased weight average molecular weight esterified cellulose ether is produced in a process for reacting cellulose ether with aliphatic monocarboxylic anhydride and dicarboxylic anhydride, the process comprising: a) cellulose ether, dicarboxylic anhydride And b) heating the reaction mixture to a temperature between 60 ° C. and 110 ° C. before, during or after mixing the components of the reaction mixture； Adding the acid anhydride to the reaction mixture of step a) in succession or at least in three portions to complete the reaction. [Selection figure] None
